2. Description of the Related Art
Widely used conventional spark plugs include an insulator having a through hole in an axial direction of the spark plug and which comprises alumina ceramics, a center electrode partially inserted in a front end of the through hole, an external terminal partially inserted in a rear end of the through hole, and a conductive seal provided between the external terminal and the center electrode in the through hole.
In such a spark plug, it is known (in reference to JP-A-2003-22886 corresponding to U.S. Pat. No. 6,744,189, for example) that compression stress on the conductive seal prevents cracking and peeling at the interface between the conductive seal and the insulator. To achieve this effect, the conductive seal is proposed to contain an inorganic material having a thermal expansion coefficient lower than that of alumina constituting the insulator, such as an insulating filler composed of β-eucriptite, β-spodumene, keatite, silica, mullite, cordierite, zircon and aluminum titanate, so that the conductive seal assumes a smaller thermal expansion coefficient than that of the insulator.
3. Problems to be Solved by the Invention
However, the conductive seal containing the insulating filler as described above results in an increased amount of solid components at the time when the base glass in the conductive seal is softened, and thereby causes increased hardness of the conductive seal as a whole. While press-fitting an external terminal against the conductive seal, the conductive seal is heated so as to soften the base glass, and then cooled so as to seal and fix the external terminal and the center electrode with the conductive seal (hereinafter also called a “glass sealing process”). In this process, the aforementioned conductive seal can be too hard to apply a sufficient sealing load to the external terminal, thus causing so-called “terminal misalignment”, in which the external terminal is not sufficiently inserted into the insulator. If the sealing load is simply increased, on the other hand, the insulator may break when the external terminal is press-fitted in the insulator.